Patrick Thomassey, defense attorney for Alex Hribal, does a live television interview outside Franklin Regional High School, Thursday morning, April 10, 2014, Murrysville, Pa. Authorities have charged Hribal, 16, with four counts of attempted homicide and 21 counts of aggravated assault in a stabbing attack at the school on April 9. (AP Photo/Tribune Review, Sean Stipp) PITTSBURGH OUT

PITTSBURGH — The 16-year-old boy accused of stabbing 22 people at his high school was dazed “like a deer in the headlights” hours later and doesn’t fully grasp what he did, his attorney said Thursday as he sketched out the beginnings of a possible mental health defense.

Deepening the mystery of what set off the violence, attorney Patrick Thomassey said Alex Hribal had no history of mental illness or troublemaking, didn’t abuse drugs and wasn’t an outcast at school, where the lawyer described him as a B or B-plus student.

“In a case like this, it’s pretty obvious to me that there must be something inside this young man that nobody knew about,” Thomassey said.

The local prosecutor, meanwhile, said Hribal remained an enigma.

“We have very little information about him,” said Westmoreland County District Attorney John Peck, “except for the fact that he was a student, his age and how he was as a student.”

Authorities seized the family’s computer as they searched for clues to Wednesday’s rampage at Franklin Regional High, about 15 miles from Pittsburgh. Authorities said Hribal armed himself with two kitchen knives and stabbed 21 students and a security guard before an assistant principal tackled him.

The slender, dark-haired boy, who looks younger than his years, was jailed without bail on four counts of attempted homicide and 21 counts of aggravated assault. Authorities are prosecuting him as an adult, but Thomassey said he will try to have the case moved to juvenile court.

He said he plans to get his client examined by a psychiatrist before a preliminary hearing April 30.

“I think his mental state now is unstable. I’m not sure that he recognizes the enormity, if that’s the word, of what has occurred,” Thomassey said.

The attack seemingly came out of nowhere, the attorney said.

“Both parents are good parents. They’re parents who pay attention to their kids, who eat dinner with their kids every day, who understand their kids’ friends, who, you know, care about who they hang out with,” Thomassey said.